The present invention relates to internal combustion engine control systems and methods. More particularly, the present invention relates to systems and methods for improving the efficiency of internal combustion engines utilizing advanced variable displacement.
Current internal combustion engines are notoriously inefficient. Such inefficiency results in smaller distances required between fill ups, increased costs of operation and the release of greater amounts of undesirable emissions, including greenhouse gasses, as compared to more efficient engine types. With increases in fuel costs, and with ever more environmentally conscious consumers and legislation, it has become imperative to provide increasingly fuel efficient engines for automotive, industrial and other applications.
In regards to the automotive industry, a number of methods have developed in order to increase engine efficiency. Some measures are subtle, such as optimizing gearing ratios. On the other extreme is the introduction of hybrid systems which combine electric engines and complicated drive train systems with the internal combustion engine.
Overall, such fuel saving measures have improved the efficiency of vehicles. However, there is often a tradeoff of fuel saving features to performance. Also, in regards to some of the more extreme fuel saving features, such as hybrid systems, conversion of existing internal combustion engine vehicles is difficult and often financially prohibitive. Moreover, few of the current measures utilized to increase engine efficiency actually address the root cause of the inefficiencies.
Most internal combustion engines utilize reciprocating pistons with two or four stroke working cycles. These engines operate at efficiencies that are far below theoretical peak efficiency because the engine must be able to operate under a wide variety of loads. Accordingly, the amount of air and/or fuel that is delivered into each cylinder typically varies depending upon the desired torque or power output. It is well understood that the cylinders are more efficient when they are operated under specific conditions that permit full or near-full compression and optimal fuel injection levels that are tailored to the cylinder size and operating conditions. Generally, the best thermodynamic efficiency of an engine is found when the most air is introduced into the cylinders, which typically occurs when the air delivery to the engine is un-throttled. However, in engines that control the power output by using a throttle to regulate the flow of air into the cylinders (e.g., Otto cycle engines used in many cars), operating at an un-throttled position (i.e., at “full throttle”) would typically result in the delivery of more power (and often far more power) than desired or appropriate.
In engines that do not generally throttle the flow of air into the cylinders (e.g., most diesel engines), power is controlled by modulating the amount of fuel delivered to the cylinders. Operating such engines at thermodynamically optimal fuel injection levels, again, would typically result in the delivery of more power than desired or appropriate. Therefore, in most applications, standard internal combustion engines are operated under conditions well below their optimal thermodynamic efficiency a significant majority of the time.
There are a number of reasons that internal combustion engines do not operate as efficiently at partial throttle. One of the most significant factors is that less air is provided to the cylinder at partial throttle than at full throttle which reduces the effective compression of the cylinder, which in turn reduces the thermodynamic efficiency of the cylinder. Another very significant factor is that operating at partial throttle requires more energy to be expended to pump air into and out of the cylinders than is required when the cylinder is operating at full throttle—these losses are frequently referred to as pumping losses.
One approach to gain engine efficiency, and hence lower fuel consumption is varying the displacement of the engine (variable displacement). The concept was initially introduced by Cadillac™ in the 1980s to less than stellar reviews. The luxury sedan's engine flipped between four, six and eight cylinders, but had a poor service record and suffered from engine vibration issues. The greatest vibration problems kicked in when the engine was in six-cylinder mode, an intrinsically unbalanced configuration. The technology was deemed too unstable, with too little demand to be developed until recently.
Most current commercially available variable displacement engines effectively “shut down” some of the cylinders during certain low-load operating conditions. When a cylinder is “shut down”, its piston still reciprocates, however neither air nor fuel is delivered to the cylinder so the piston does not deliver any power during its power stroke. Since the cylinders that are shut down do not deliver any power, the proportionate load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate at an improved thermodynamic efficiency. The improved thermodynamic efficiency results in improved fuel efficiency. Although the remaining cylinders tend to operate at improved efficiency, they still do not operate at their optimal efficiency the vast majority of the time because they are still not operating consistently at “full throttle.” That is, they have the same drawbacks of partial throttle operations, (e.g., lower compression, higher pumping losses) even though the scale of their inefficiencies is reduced.
Another drawback of most current commercially available variable displacement engines is that they tend to revert out of the variable displacement mode very quickly when changes are made to the desired operational state of the engine. For example, many commercially available automotive variable displacement engines appear to revert to a “conventional” all cylinder operational mode any time the driver requests non-trivial additional power by further depressing the accelerator pedal. In many circumstances this results in the engine switching out of the fuel saving variable displacement mode, even though the engine is theoretically capable of delivering the desired power using only the reduced number of cylinders that were being used in the variable displacement mode. It is believed that the reason that such variable displacement engines kick out of the variable displacement mode so quickly is due to the perceived difficulty of controlling the engine to provide substantially the same response regardless of how many cylinders are being used at any given time.
More generally, engine control approaches that vary the effective displacement of an engine by skipping the delivery of fuel to certain cylinders are often referred to as “skip fire” control of an engine. In skip fire control, fuel is not delivered to selected cylinders based on some designated control algorithm. The variable displacement engines that effectively shut down cylinders that are described above are essentially a class of skip fire engines. Over the years, a number of skip fire engine control arrangements have been proposed, however, most still contemplate throttling the engine or modulating the amount of fuel delivered to the cylinders in order to control the engine's power output.
As suggested above, most commercially available variable displacement engines shut down specific cylinders to vary the displacement in discrete steps. Other approaches have also been proposed for varying the displacement of an engine to facilitate improved thermodynamic efficiency. For example, some designs contemplate varying the effective size of the cylinders to vary the engine's displacement. Although such designs may improve thermodynamic and fuel efficiencies, existing variable cylinder size designs tend to be relatively complicated and expensive to produce making them impractical for widespread use in commercial vehicles.
Although existing variable displacement engines work well in many applications, there are continuing efforts to further improve the thermodynamic efficiency of internal combustion engines without necessarily requiring expensive alterations to the engine's design.
Furthermore, in many existing engines used by current vehicles, the number of cylinders capable of being “shut down” when in variable displacement modes may be limited. For example, in the GM Generation IV small block V8 engine half of the cylinders are capable of being shut down. Current engine design, however, requires that the remaining 4 cylinders remain operational. Likewise, the six cylinder Honda™ J-series engine is capable of shutting down 3 or 4 cylinders. Yet other engines are arranged to shut down cylinders in banks. In virtually all such engines, current methods of variable displacement modes are unable to provide low power one or two cylinder operation.
Moreover, it is usual for existing variable displacement style systems to readily drop out of a variable displacement mode and revert to “conventional” operation when slightly increased power demands are placed upon them. Thus, the time spent in a reduced displacement mode is typically limited, resulting in reduced efficiency realization.
Also, current variable displacement type systems have relatively slow reaction times, thus the switching between cylinders being used is less responsive than required for peak efficiencies. Such slow modulation results in the engines being run in an overpowered manner so that there are no perceived performance issues. Again, this results in the engine being run at a lower than peak efficiency rating.
Additionally, variable displacement modes may result in strains being placed upon the exhaust systems of the engines. This may result in the engines failing compliance to state and federal emissions standards. A catalytic converter may be used to reduce the toxicity of emissions from an internal combustion engine. The catalytic converter provides an environment for a chemical reaction wherein toxic combustion by-products are converted to less-toxic substances. There are two way and three way catalytic converters.
The three reactions in a three way catalytic converter occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. This is between 14.6 and 14.8 parts air to 1 part fuel, by weight, for gasoline. The ratio for LPG, natural gas and ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels. Generally, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system employing one or more oxygen sensors.
When a 3-way catalyst is used in an open-loop system, NOx reduction (one of the three reactions) efficiency is low. Within a narrow fuel/air ratio band surrounding stoichiometry, conversion of all three pollutants is very complete, sometimes approaching 100%. However, outside of that band, conversion efficiency falls off very rapidly. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and hydrocarbons) are favored, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NOx is favored, at the expense of CO and HC oxidation.
In many catalytic converters, the oxygen sensor is the basis of the closed loop control system on a spark ignited rich burn engine, however it is also used for diagnostics. In vehicles with OBD II, a second oxygen sensor is fitted after the catalytic converter to monitor the O2 levels. The on-board computer makes comparisons between the readings of the two sensors. If both sensors give the same output, the catalytic converter is not functioning and must be replaced.
During variable displacement operation, readings by the oxygen sensors may be erratic and may signify “improper” engine operation. In reality, such oxygen readings may be perfectly acceptable, or may require changes to air fuel ratios to ensure proper operation. Regardless, being in compliance with emissions standards is imperative for most automotive engines, and has resulted in hurdles to the development of successful wide range variable displacement systems.
In view of the foregoing, systems and methods for improving efficiency in internal combustion engines are disclosed. The present invention provides a novel system for variable displacement engine control whereby existing engines which include cylinders incapable of being “shut down” may be easily modified to provide a full range of variable displacement modes, including very low power operations using one or more operational cylinders. Additionally, the present invention provides for variable displacement modes of operation which may effectively suppress undesired engine vibration.